Accurate reservoir characterization is a prerequisite for optimized oil and gas reservoir management, especially accurate estimation of in situ stress changes and pore pressure changes in the reservoir. Reservoir depletion or fluid injection into reservoir during hydrocarbon production or gas storage (natural or CO2) leads to changes in reservoir pore pressure. Consequently, these reservoir pore pressure variations affect the reservoir stressfield that leads to alteration of petrophysical properties of reservoir rock such as porosity and permeability, strength, compressibility, etc. Furthermore, issues such as subsidence, borehole stability problems during drilling, casing collapse, sand production and reactivation of reservoir-bounding and sealing faults are strongly associated with stress changes within the reservoir. In naturally fractured reservoirs, the effects of stress changes on reservoir behavior are even more pronounced, particularly with respect to hydrocarbon fluid flow through fractures. Therefore, knowledge of in situ stresses and accurate prediction of changes in stress state due to reservoir depletion and fluid injection is important in oil and gas reservoir characterization, enhanced oil recovery projects, and reservoir management.
In order to quantify the in situ stress changes due to reservoir depletion or fluid injection into the reservoir, filed data in combination with either an analytical technique or a numerical approach may be utilized. The analytical techniques are used to understand the coupling of stress components and pore pressure (i.e., stress path parameter). In analytical models, the vertical stress or lithostatic stress is assumed to be constant during the reservoir depletion with no strain in the horizontal directions. The analytical models, which are currently used to predict the stress path of reservoir during production, are based on the theory of linear poroelasticity. Assuming a uniaxial strain condition, different models have been developed for isotropic, transversely isotropic, and orthotropic rock behavior. Despite relatively extensive analytical and numerical investigations of depletion effects on reservoir stress state, very limited experimental data is available on the coupling of pore pressure drawdown and reservoir stress components.
Only limited depletion tests are conducted under hydrostatic loading or conventional triaxial compression conditions due to the complexity of the experimental programs and apparatus capable of depletion simulation under true triaxial stress loading. Conventional triaxial compression conditions, which assume that both horizontal stress components are equal, are a rather simplified representation of the general in situ stress state and are not fully representative of the actual ground conditions. A true triaxial stress state that simulates the realistic crustal conditions would be desirable in quantifying the in situ stress changes due to reservoir depletion or fluid injection into the reservoir. There is further a need for simulating the reservoir depletion or fluid injection into the reservoir under true triaxial stress loading condition and a uniaxial strain boundary (constant vertical stress and zero minimum and maximum horizontal strains) condition.